This invention relates to a solution for treating a surface of an iron material to form a coating thereon, a method for treating a surface of an iron material using such solution, and an iron material having a coating formed by using such method.
The surface treatments most commonly used for depositing a coating having excellent corrosion resistance after coating on a metal surface are zinc phosphate treatment and chromate treatment. Among these, the zinc phosphate treatment is capable of depositing a coating having excellent corrosion resistance on a steel strip such as a hot rolled steel strip or a cold rolled steel strip or a galvanized steel strip.
However, this zinc phosphate treatment inevitably involves generation of sludge as a by-product of the reaction.
In view of such situation, technologies have been recently developed in which the corrosion resistance is imparted to a metal surface by covering the metal surface with a thin coating containing a metal such as zirconium, the solution used in the surface treatment is free from harmful components, and the sludge generation is avoided. Exemplary such surface treatment methods are as described below.
For example, JP 2000-204485 A discloses a compound containing nitrogen atom having a lone pair, and a chromium-free coating composition for a metal surface containing such compound and a zirconium compound. An object of this chromium-free coating composition for a metal surface is to provide a coating having improved corrosion resistance and adhesion after further coating by using a solution free from the harmful hexavalent chromium.
However, the metal materials which can be treated by this chromium-free coating composition for a metal surface are limited to aluminum alloys, and it has been difficult to treat an article having a surface with complicated shape because a coating is formed by applying the coating composition on the article and subsequently drying the article.
JP 2-25579 A discloses a surface treatment for depositing a coating having excellent adhesion and corrosion resistance after further coating by chemical conversion. This treatment uses a surface treating composition and a surface treating bath containing cerium, zirconium, phosphoric acid, and fluorine compound.
However, this surface treatment is also limited in the material which can be treated, and as in the case of the chromium-free coating composition for a metal surface described in JP 2000-204485 A, only aluminum or aluminum alloys which are inherently corrosion resistant can be treated by this method, and this surface treatment cannot deposit a coating on the surface of an iron material (an iron-base metal material).
JP 2000-199077 A discloses a surface treatment in which a coating having excellent corrosion resistance and adhesion after further coating is deposited by using a surface treating composition containing a metal acetylacetonate and a water soluble inorganic titanium compound or a water soluble inorganic zirconium compound. This method expanded the range of metal materials that could be surface treated from only the aluminum alloys to the range also including magnesium, magnesium alloys, zinc, and galvanizing alloys.
However, a coating cannot be deposited by this surface treatment on the surface of an iron material such as a hot rolled steel strip or a cold rolled steel strip.
JP 5-195244 A discloses a surface treatment of a metal surface by a chromium-free acidic composition. In this treatment of the metal surface, an aqueous solution containing the components which can form a corrosion-resistant coating is applied on the metal surface, and without rinsing with water, the coating is fixed by baking and drying. Accordingly, the coating formation does not involve a chemical reaction, and the method can also be used in depositing a coating on a metal surface as in a hot rolled steel strip, a cold rolled steel strip, a galvanized steel strip, and an aluminum alloy plate.
However, as in the case of the chromium-free coating composition for a metal surface described in JP 2000-204485 A, the coating is formed by application and drying, and formation of a uniform coating on an article having a complicated surface shape has been difficult.
JP 2004-43913 A discloses a metal chemical conversion method using a bath containing zirconium ion and/or titanium ion and fluorine ion. This method can be used for treating metal materials including iron base material, aluminum, and zinc.
However, this method has various limitations in the treatment process including the need of controlling the iron ion concentration in the chemical conversion agent with an oxidizing agent during the treatment.
As described above, no surface treating method has been conventionally known which uses a surface treating solution free from environmentally harmful ingredients and is capable of providing a coating having an excellent corrosion resistance simultaneously with an excellent adhesion on an iron material at an acceptable production efficiency.